1. Field of the Invention
This invention relates in general to semiconductor memory devices, such as Static Random Access Memory (SRAM) devices. More specifically, the invention relates to dummy wordlines that control the timing of the firing of sense amplifiers in memory devices in relation to the firing of wordlines in the memory devices.
2. State of the Art
As shown in FIG. 1, a conventional SRAM (not shown in its entirety) includes wordline selection and driving circuitry 10 that initiates the process of reading a logic bit from an SRAM cell 12 when a timing control 14 causes the circuitry 10 to activate a wordline 16, thereby activating the SRAM cell 12. The activated SRAM cell 12 dumps its logic bit onto bitlines 18 and 20 by inducing a voltage differential between the bitlines 18 and 20 representative of the logic bit. The timing control 14 then causes a sense amplifier (not shown) in column selection and sensing circuitry 22 to sense and amplify the voltage differential between the bitlines 18 and 20. The amplified voltage differential, still representative of the logic bit output from the SRAM cell 12, is then output through output buffers (not shown) for use by external circuitry (not shown).
The timing of the xe2x80x9cfiringxe2x80x9d (i.e., the activation) of the sense amplifier in the column selection and sensing circuitry 22 in relation to the firing of the wordline 16 is important. When the sense amplifier is fired too early, the activated SRAM cell 12 may not have sufficient time to induce a voltage differential on the bitlines 18 and 20 that can be sensed by the sense amplifier. As a result, the sense amplifier may incorrectly sense a logic xe2x80x9c0xe2x80x9d bit, for example, on the bitlines 18 and 20 while the SRAM cell 12 is in the process of dumping a logic xe2x80x9c1xe2x80x9d bit onto the bitlines 18 and 20. When the sense amplifier is fired too late, the SRAM cell 12 is no longer activated, so, again, the sense amplifier may sense the wrong logic bit on the bitlines 18 and 20. Even when the sense amplifier is fired in a xe2x80x9cwindowxe2x80x9d of time between xe2x80x9ctoo earlyxe2x80x9d and xe2x80x9ctoo late,xe2x80x9d if the sense amplifier is fired toward the end of this window, time and power are wasted keeping the wordline 16 and the SRAM cell 12 activated until the end of the window. Thus, the sense amplifier is preferably fired in the beginning or the middle of the window of time between too early and too late in order to increase the speed with which logic bits are read from the SRAM cell 12 and to limit power usage.
Unfortunately, the timing control 14 does not always provide this preferred timing, because process variations during fabrication can vary the impedance-induced signal propagation delay along the wordline 16, and thus the timing of the activation of the SRAM cell 12, in a way that cannot be anticipated by the timing control 14. As a result, the timing control 14 is typically designed with extra delay time between the firing of the wordline 16 and the firing of the sense amplifier in the column selection and sensing circuitry 22 to correct for those wordline signal propagation delays that fall within a typical range. Although this approach creates a working part, it does not enhance the speed of the part or reduce its power usage, as is desired.
The timing problem described above also arises in Dynamic RAM (DRAM) devices, and it has been addressed in a DRAM device 24 shown in FIG. 2 manufactured by the Assignee of the present invention, Micron Technology, Inc. of Boise, Id. In the DRAM device 24, the timing of the firing of column selection and sensing circuitry 26 in relation to the firing of wordlines (not shown) in an array 28 of DRAM memory cells (not shown) by wordline selection and driving circuitry 30 is controlled by a folded delay line 32 fabricated in the periphery 34 of the DRAM device 24. The length of the delay line 32 is selected so a timing signal output by the circuitry 30 at the same time a wordline in the array 28 is fired propagates along the delay line 32 and arrives at the circuitry 26 in time to fire the circuitry 26 at the preferred time described above.
Again, process variations during fabrication vary the impedance-induced signal propagation delay of the folded delay line 32, and of the wordlines in the array 28, in such a way that the delay line 32 must include extra delay length to accommodate variations in the propagation delays that fall within a typical range, in much the same way that the timing control 14 of FIG. 1 must include extra delay time. As a result, the delay line 32 is not a reliable means of achieving the preferred timing described above, and thus does not reliably meet the goals of increased speed and reduced power usage.
Therefore, there is a need in the art for an improved device and method for controlling the timing of the firing of sense amplifiers and other column selection and sensing circuitry in memory devices, such as SRAM""s and DRAM""s, in relation to the firing of wordlines in such memory devices. Such an improved device and method should accommodate normal process variations that vary signal propagation delays in such memory devices while increasing speed and limiting power usage.
An apparatus in accordance with the present invention controls the timing of the firing of column selection and sensing circuitry, such as sense amplifiers, in a semiconductor memory, such as a Static Random Access Memory (SRAM) or a Dynamic RAM (DRAM). The apparatus includes a dummy wordline fabricated within an array of memory cells in the semiconductor memory. The dummy wordline has a length selected so the dummy wordline can delay a timing signal traversing its length by a selected amount of time before the timing signal fires the column selection and sensing circuitry.
By selecting a suitable length for the dummy wordline, the timing of the firing of the column selection and sensing circuitry can be controlled. Also, the dummy wordline preferably receives the timing signal from wordline driving circuitry at the same time the wordline driving circuitry fires an active wordline in the array of memory cells so the apparatus of the present invention controls the timing of the firing of the column selection and sensing circuitry in relation to the firing of active wordlines in the semiconductor memory.
Because the dummy wordline is fabricated within the array of memory cells, fabrication process variations alter the impedance characteristics of the dummy wordline in relatively the same way as they alter the impedance characteristics of active wordlines in the array. As a result, the time delay associated with the active wordlines varies in the same manner as the time delay associated with the dummy wordline, so no delay time need be built into the dummy wordline to accommodate variations in the time delay associated with the active wordlines. As a result, the length of the dummy wordline can be selected to optimize the timing of the firing of the column selection and sensing circuitry so it occurs at a preferred point in a window of time between too early and too late, thus reducing power usage by minimizing the active time of wordlines and increasing the speed of the semiconductor memory by optimizing the timing of its column selection and sensing circuitry.
In other embodiments of the present invention, a semiconductor memory, an electronic system, and a semiconductor wafer include the inventive dummy wordline described above.
In an inventive method of the present invention, the timing of the firing of column selection and sensing circuitry in a semiconductor memory is controlled by first generating a timing signal for firing the circuitry. Arrival of the timing signal at the column selection and sensing circuitry is then delayed by impeding conduction of the signal to the circuitry with impedance characteristics that are substantially the same as impedance characteristics of an active wordline traversing an array of memory cells in the semiconductor memory. The column selection and sensing circuitry is then fired using the delayed timing signal. As previously described, the timing signal is preferably delayed using a dummy wordline that traverses the array of memory cells of the semiconductor memory.